The present invention relates to a catalyst for the oxidation of ethane and/or ethylene to acetic acid and to a process for the production of acetic acid utilising the aforesaid catalyst.
Catalysts and processes for the production of acetic acid by the oxidation of ethane and ethylene are known in the art from, for example, U.S. Pat. No. 4,250,346; EP-A-0407091; DE-A-19620542; and DE-A-19630832.
U.S. Pat. No. 4,250,346 discloses the oxidative dehydrogenation of ethane to ethylene in a gas phase reaction at relatively high levels of conversion, selectivity and productivity at a temperature less than 500xc2x0 C. using as catalyst a composition comprising the elements molybdenum, X and Y in the ratio
MoaXbYc
wherein
X is Cr, Mn, Nb, Ta, Ti, V and/or W, and preferably Mn, Nb, V and/or W
Y is Bi, Ce, Co, Cu, Fe, K, Mg, Ni, P, Pb, Sb, Si, Sn, Tl and/or U, and preferably Sb, Ce and/or U,
a is 1,
b is 0.05 to 1.0 and
c is 0 to 2, and preferably 0.05 to 1.0, with the proviso that the total value of c for Co, Ni and/or Fe is less than 0.5.
EP-A-0407091 discloses a process for the production from gaseous ethane and/or ethylene of a product comprising ethylene and/or acetic acid, by contacting the ethane and/or ethylene and a molecular oxygen-containing gas at elevated temperature with a calcined molybdenum-containing ethane oxidative dehydrogenation catalyst composition characterised in that molybdenum in the oxidative dehydrogenation catalyst composition is replaced in whole or in part by either rhenium or a combination of rhenium and tungsten.
Also disclosed in EP-A-0407091 is a catalyst comprising the elements A, X and Y in combination with oxygen, the gram-atom ratios of the elements A:X:Y being a:b:c,
wherein A=ModReWf,
X=Cr, Mn, Nb, Ta, Ti, V and/or W, and preferably Mn, Nb, V and/or W,
Y=Bi, Ce, Co, Cu, Fe, K, Mg, Ni, P, Pb, Sb, Si, Sn, Tl and/or U, and preferably Sb, Ce and/or U,
a=1,
b=0 to 2, preferably 0.05 to 1.0,
c=0 to 2, preferably 0.001 to 1.0, and more preferably 0.05 to 1.0 with the proviso that the total value of c for Co, Ni, and/or Fe is less than 0.5,
d+e+f=a,
d is either zero or greater than zero,
e is greater than zero, and
f is either zero or greater than zero.
DE-A-19620542 discloses a catalyst for the selective oxidation of ethane and/or ethylene to acetic acid containing the elements Mo, Pd, Re, X and Y in the gram atom ratios a:b:c:d:e in combination with oxygen
MoaPdbRecXdYexe2x80x83xe2x80x83(I)
where the symbols X, Y have the following signification:
X=Cr, Mn, Nb, B, Ta, Ti, V and/or W
Y=Bi, Ce, Co, Cu, Te, Fe, Li, K, Na, Rb, Be, Mg, Ca, Sr, Ba, Ni, P, Pb, Sb, Si, Sn, Tl and/or U;
the indices a, b, c, d and e stand for the gram atom ratios of the corresponding elements, where a=1, b greater than 0, c greater than 0, d=0.05 to 2 and e=0 to 3. Also disclosed in DE-A-19620542 is a process for the selective production of acetic acid from a gaseous charge of ethane, ethylene or mixtures thereof in addition to oxygen by bringing the gaseous charge into contact with a catalyst of the formula (I).
Finally, DE-A-19630832 discloses a catalyst for the selective oxidation of ethane, ethylene or mixtures thereof as well as oxygen, containing the elements Mo, Pd, X and Y in the gram ratios a:b:c:d in combination with oxygen
MoaPdbXcYdxe2x80x83xe2x80x83(I)
where the symbols X, Y have the following signification:
X stands for one or more of the elements selected from the group Cr, Mn, Nb, Ta, Ti, V and W;
Y stands for one or more of the elements selected from the group B, Al, Ga, In, Pt, Zn, Cd, Bi, Ce, Co, Rh, Ir, Cu, Ag, Au, Fe, Ru, Os, K, Rb, Cs, Mg, Ca, Sr, Ba, Zr, Hf, Ni, P, Pb, Sb, Si, Sn, Tl, and U;
the indices a, b, c, d stand for the gram atom ratios of the corresponding elements, where
a=1; b greater than 0; c greater than 0 and d=0-2. Also disclosed in DE-A-19630832 is a process for the selective production of acetic acid from a gaseous charge of ethane; ethylene or mixtures thereof in addition to oxygen by contacting the gaseous charge with a catalyst of the formula (I).
International patent publication WO 98/47850 published after the priority date of the present application relates to a process and catalyst for preparing acetic acid by catalytic oxidation of ethane. The catalyst used has the formula WaXbYcZd in which X stand for one or more elements selected from the group Pd, Pt, Ag and/or Au; Y stands for one or more elements selected from the group V, Nb, Cr, Mn, Fe, Sn, Sb, Cu, Zn, U, Ni and/or Bi; Z stands for one or more elements selected from the group Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, Ba, Sc, Y, La, Ti, Zr, Hf, Ru, Os, Co, Rh, Ir, B, Al, Ga, In, Tl, Si, Ge, Pb, P, As and/or Te in the formula a=1, b is greater than 0, c is greater than 0 and d is a number from 0 to 2. Tungsten is thus an essential component of the catalyst.
U.S. Pat. No. 5,750,777 (equivalent to EP-A-719756) relates to production of acetic acid by oxidation of ethane in the presence of a catalyst in which the active phase comprises vanadium, titanium, molybdenum, phosphorus and oxygen which includes a dopant from the following elements: K, Rb, Cs, Ca, Mg, Zr, Hf, Nb, Ta, Cr, W, Mn, Re, Fe, Ru, Os, Rh, Ir, Ni, Pd, Cu, Ag, Zn, Cd, Ti, Si, Ge, Sn, As, Sb, Bi, Ga and the rare earths. However, there are no specific examples containing silver or iridium.
U.S. Pat. No. 4,568,790 relates to a process for the low temperature catalytic oxydehydrogenation of ethane to ethylene in a gas phase using a catalyst having a calcined composition of MoaVbNbcSbd wherein a=0.5 to 0.9, b=0.1 to 0.4, c=0.001 to 0.2 and d=0.001 to 0.1.
U.S. Pat. No. 4,596,787 relates to a process for preparing a supported catalyst for the low temperature oxydehydrogenation of ethane to ethylene in a gas phase, including catalysts having a calcined composition containing MoaVbNbcSbdXe wherein X=nothing or at least one of the following: Li, Sc, Na, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, Y, Ta, Cr, Fe, Co, Ni, Ce, La, Zn, Cd, Hg, Al, Tl, Pb, As, Bi, Te, U, Mn and W, a=0.5 to 0.9, b=0.1 to 0.4, c=0.001 to 0.2, d=0.001 to 0.1 and e=0.001 to 1.0 for X equal to at least one element and e=0 for X=0.
There remains a need for a catalyst for the selective oxidation of ethane and/or ethylene to acetic acid and a process for the selective production of acetic acid employing the catalyst. We have found that oxidation catalysts employing silver and/or iridium as an essential component can fulfill the need for a selective oxidation catalyst and process employing same.
Accordingly, the present invention provides a catalyst composition for the selective oxidation of ethane and/or ethylene to acetic acid which composition comprises in combination with oxygen the elements:
Moa.Wb.Agc.Ird.Xe.Yfxe2x80x83xe2x80x83(I)
wherein X is the elements Nb and V;
Y is one or more elements selected from the group consisting of:
Cr, Mn, Ta, Ti, B, Al, Ga, In, Pt, Zn, Cd, Bi, Ce, Co, Rh, Cu, Au, Fe, Ru, Os, K, Rb, Cs, Mg, Ca, Sr, Ba, Zr, Hf, Ni, P, Pb, Sb, Si, Sn, Tl, U, Re and Pd;
a, b, c, d, e and f represent the gram atom ratios of the elements such that
0 less than axe2x89xa61, 0xe2x89xa6b less than 1 and a+b=1;
0 less than (c+d)xe2x89xa60.1;
0 less than exe2x89xa62; and
0xe2x89xa6fxe2x89xa62.
Catalysts embraced within the formula (I) include:
Moa.Wb.Agc.Xe Yf 
Moa.Wb.Ird.Xe Yf 
Moa.Wb.[Ag+Ir]c+d.Xe.Yf 
Moa.Agc.Xe.Yf 
Moa.Ird.Xe.Yf 
Moa.[Ag+Ir]c+d.Xe.Yf 
[Mo+W]a+b.Age.Xe.Yf 
[Mo+W]a+b.Ird.Xe.Yf 
[Mo+W]a+b.[Ag+Ir]c+d.Xe.Yf 
Examples of suitable catalysts having the formula (I) include:
(i) Mo0.37.Ag0.01.Re0.25.V0.26.Nb0.07.Sb0.03. Ca0.02.Oyxe2x80x2 which renormalised on the basis of Mo is the same as Mo1.00.Re0.069.V0.72.Nb0.25.Sb0.08. Ca0.03.Ag0.028Oy;
(ii) Mo0.37.Ir0.01.Re0.25.V0.26.Nb0.07.Sb0.03. Ca0.02.Oyxe2x80x2 which renormalised on the basis of Mo is the same as Mo1.00.Re0.69.V0.72.Nb0.25.Sb0.08. Ca0.03.Ir0.028Oy;
(iii) Mo1.00.V0.25.Nb0.12.Ag0.014Oy; and
(iv) Mo1.00.V0.25.Nb0.12.Ag0.000028.Ir0.0000018Oy.
wherein yxe2x80x2 and y are numbers which satisfy the valencies of the elements in the composition for oxygen.
An advantage of catalyst compositions according to the present invention is that they can be more active and selective in converting ethane and/or ethylene to acetic acid.
Preferably, silver and/or iridium is present in an effective amount such that c+d is at least 10xe2x88x926. Preferably, c and d are such that (c+d)xe2x89xa60.05. Silver is more effective than iridium. Preferably, more silver is present than iridium on a gram atom basis. Preferably, c and d are such that c is at least 10 times greater than d.
Preferably, e is at least 0.05, more preferably e is at least 0.2. Preferably, e is no greater than 0.5. Yet more preferably, e is such that 0.05xe2x89xa6exe2x89xa60.5. Still more preferably, e is such that 0.2xe2x89xa6exe2x89xa60.5.
Preferably, f is at least 10xe2x88x926. Preferably, f is no greater than 0.2. Yet more preferably, f is such that 10xe2x88x926xe2x89xa6fxe2x89xa60.2.
Preferably, Y is at least one element selected from the group consisting of Cu, Pd, Pt, Re, Ru and Sb.
Preferably, a is at least 0.1. More preferably, a is at least 0.5. Preferably, b is no greater than 0.9.
The catalyst compositions may be prepared by any of the methods conventionally employed for the preparation of catalysts. Suitably the catalyst may be prepared from a solution of soluble compounds and/or complexes and/or compounds of each of the metals. The solution is preferably an aqueous system having a pH in the range from 1 to 12, preferably from 2 to 8, at a temperature of from 20xc2x0 to 100xc2x0 C.
Generally, a mixture of compounds containing the elements is prepared by dissolving sufficient quantities of soluble compounds and dispersing any insoluble compounds so as to provide a desired gram-atom ratio of the elements in the catalyst composition. The catalyst composition may then be prepared by removing the solvent from the mixture. The catalyst may be calcined by heating to a temperature of from 200 to 550xc2x0 C., suitably in air or oxygen, for a period of from 1 minute to 24 hours. Preferably, the air or oxygen is slowly flowing.
The catalyst may be used unsupported or supported. Suitable supports include silica, alumina, zirconia, titania, silicon carbide and mixtures of two or more thereof.
Further details of a suitable method for preparing a catalyst composition may be found in, for example, EP-A-0166438.
The catalyst may be used in the form of a fixed or a fluidised bed.
In another embodiment the present invention provides a process for the production of acetic acid from a gaseous mixture comprising ethane and/or ethylene which process comprises contacting the gaseous mixture with a molecular oxygen-containing gas at elevated temperature in the presence of a catalyst composition as hereinbefore described.
The feed gas comprises ethane and/or ethylene, preferably ethane.
Ethane and/or ethylene may be used in substantially pure form or admixed with one or more of nitrogen, methane, carbon dioxide and water in the form of steam, which may be present in major amounts, for example greater than 5 volume percent or one or more of hydrogen, carbon monoxide, C3/C4 alkenes and alkenes, which may be present in minor amounts, for example less than 5 volume percent.
The molecular oxygen-containing gas may be air or a gas richer or poorer in molecular oxygen than air, for example oxygen. A suitable gas may be, for example, oxygen diluted with a suitable diluent, for example nitrogen.
It is preferred to feed, in addition to ethane and/or ethylene and the molecular oxygen-containing gas, water (steam) because this can improve the selectivity to acetic acid.
The elevated temperature may suitably be in the range from 200 to 500xc2x0 C., preferably from 200 to 400xc2x0 C.
The pressure may suitably be atmospheric or superatmospheric, for example in the range from 1 to 50 bar, preferably from 1 to 30 bar.
The catalyst composition is preferably calcined before use in the process of the invention. Calcination may suitably be achieved by heating at a temperature suitably in the range from 250 to 500xc2x0 C. in the presence of an oxygen-containing gas, for example air.
Operating conditions and other information applicable to the performance of the invention may be found in the aforesaid prior art, for example U.S. Pat. No. 4,250,346.
In a preferred embodiment, the oxidation catalyst of the present invention may be used in an integrated process for the production of acetic acid and/or vinyl acetate such as that described, for example, in International patent publication WO 98/05620, the contents of which are incorporated herein by reference. Thus, according to this embodiment, there is provided an integrated process for the production of acetic acid and/or vinyl acetate which comprises the steps:
(a) contacting in a first reaction zone a gaseous feedstock comprising ethylene and/or ethane and optionally steam with a molecular oxygen-containing gas in the presence of a catalyst active for the oxidation of ethylene to acetic acid and/or ethane to acetic acid and ethylene as hereinbefore described, to produce a first product stream comprising acetic acid, water and ethylene (either as unreacted ethylene and/or as co-produced ethylene) and optionally also ethane, carbon monoxide, carbon dioxide and/or nitrogen; and
(b) contacting in a second reaction zone in the presence or absence of additional ethylene and/or acetic acid at least a portion of the first gaseous product stream comprising at least acetic acid and ethylene and optionally also one or more of water, ethane, carbon monoxide, carbon dioxide and/or nitrogen with a molecular oxygen-containing gas in the presence of a catalyst active for the production of vinyl acetate to produce a second product stream comprising vinyl acetate, water, acetic acid and optionally ethylene.
Preferably the integrated process comprises the further steps of:
(c) separating the product stream from step (b) by distillation into an overhead azeotrope fraction comprising vinyl acetate and water and a base fraction comprising acetic acid; and
(d) either (i) recovering acetic acid from the base fraction separated in step (c) and optionally recycling the azeotrope fraction separated in step (c) after partial or complete separation of the water therefrom to step (c),
or (ii) recovering vinyl acetate from the azeotrope fraction separated in step (c) and optionally recycling the base fraction separated in step (c) to step (b),
or (iii) recovering acetic acid from the base fraction separated in step (c) and recovering vinyl acetate from the overhead azeotrope fraction recovered in step (c).
The catalyst and processes of the present invention will now be further illustrated by reference to the following Examples.